CFMEU state secretary Brian Parker said it's "by chance" no one was injured or killed when the crane caught fire on the construction site at the University of Technology on the corner of Broadway and Wattle Street about 9.45am on Tuesday.

Mr Parker said union officials did a full inspection of the site about two weeks ago and closed it down for four days.

"We brought up issues about leaking diesel on the crane and the dieseline leaking from the crane.

"We said to them that a maintenance regime has to take place immediately on the crane.

"That's obvious to us that it's failed to be done and we've had a situation where it's put hundreds of lives at risk."

A Lend Lease spokesman said the company was preparing a response to Mr Parker's claims.

Authorities said Broadway could remain closed until Wednesday.

Acting Superintendent Josh Turner said firefighters had not been able to access the crane cabin yet, but they believed it was diesel fuel on fire.

When fire crews arrived there were flames 10 metres high "leaping" out of the cabin 45 metres above the ground, he said.

The crane operator was able to escape by climbing down a ladder, while hundreds of workers left the site.

"There is still some fire activity from within the crane itself.

"The temperatures have dropped from when we got here from 1000 degrees to less than 100 degrees now.

"We believe what was on fire in there was the diesel fuel that essentially burnt all away."

Structural engineers will work to stabilise the crane on Tuesday afternoon before the surrounding roads can be opened.

Mr Parker said it was an "accident waiting to happen".

"If this crane was pointed out on the street ... and it caught fire there could have been hundreds of innocent bystanders killed here today.

"That jib could have collapsed onto the road. It could have been motorists, it could have been pedestrians ... It's just by chance that it wasn't facing that way when it's caught fire."

The collapse of the crane's jib, brought down scaffolding. No one was injured, but paramedics remained on scene, a NSW Ambulance Service spokeswoman said.

WorkCover NSW has announced a full investigation into the crane collapse.

A spokesperson described it as a "very serious workplace incident, and it is fortunate no one was injured or killed".

Hundreds of people remain evacuated from surrounding buildings, while the incident has caused traffic chaos in the middle of Sydney's CBD.

Earlier, hundreds of construction workers at a building across the road stopped work and were on balconies to get a view of the chaos.

A nearby office worker, Matthew McGee-Collett, said there was chaos on Broadway as police blocked traffic and office workers spilled out onto the streets. He said evacuation sirens had been going off.

"I actually heard a loud crashing noise and at first I thought a large truck had had some sort of accident, perhaps carrying some sort of load. When I went to the balcony of my office to look I could see the crane on the UTS site nearby had collapsed and there was a fire raging on it," he said.

He said part of the crane collapsed onto a building site on Wattle Street and was "hanging there crumpled against the side of the hoarding" on the site. "It's lucky the crane didn't fall on the street," he said.

City worker Sean Nunan could clearly see smoke and flames from an office building on Goulburn Street.

"The crane was really burning ... then suddenly the boom just gave way - [it] dropped as though the cable had snapped or given way," he said. "The crane continued to burn quite furiously for the next 20 minutes and even now we can still see flames."

A group of TAFE engineering students saw the crane collapse.

"One of them said 'mate, a crane's on fire' and we've seen it on fire. Then we went for another look and we saw the crane just come down and it was quite a shock," Matthew Dyson, 19, said.

"My mate joked it was going to fall and next thing we've heard this crash, bang, carnage everywhere, people running from every different direction."

The crane boom fell, crashing onto trees and scaffolding, he said.

Another student, Gus Warman, 30, said the boom appeared to give way in the heat of the fire.

"The main shaft [on the crane] was shaking for a while."

Motorists are advised to avoid the area, with Broadway closed for several blocks in both directions. Bus services using Broadway near City Road are being diverted, experiencing lengthy delays and also missing stops.